Ultraviolet (UV), vacuum ultraviolet (VUV), and extreme ultraviolet (EUV) (5-380 nm) photo-detectors have been widely used in the field of astronomy, EUV lithography, x-ray microscopy, plasma physics, UV curing and flame detection monitoring. Another relevant application is for non-line-of-sight (NLOS) communications. A wireless optical communication system relies on an optical wave with wavelength ranging from infrared, visible light, to UV to convey information. Compared with an RF system, it shows several potential advantages, such as huge unlicensed bandwidth, low-power and miniaturized transceiver, higher power densities, high resistance to jamming, and potential increase of data rate. In the wireless optics domain, infrared and UV waves are very valuable carriers.
Large unregulated bandwidth, which is virtually free of multiple access interference conditions, is commercially attractive while inherent security characteristics receive the military's attention. Similar to a radio frequency (RF) link, a typical wireless optical communication link comprises of an information source, modulator, transmitter, propagation channel, and receiver. To generate an optical source, solid state light-emitted diodes (LEDs), laser tubes and solid state laser diodes (LD) can be used. For detection, the optical receiver may comprise a lens-focusing and filtering subsystem, photo-detector (normally a photo-multiplier based on a vacuum tube), and post detection processor. Optical lenses and filters are made of selected wavelength-sensitive materials to extract the desired optical field. The photo-detector is made of photosensitive materials and produces free electrons with incident photons. Of course, a high sensitivity detector that does not use filters (to block the visible light), lens and vacuum tubes would be quite advantageous for system miniaturization.
Many UV applications use detectors with high responsiveness, very low dark current, and good radiation hardness. In astrophysics, blindness to visible solar radiation is also extremely important, especially for solar study in the EUV region, where the signals are relatively weak and the out-of-band signals, such as the visible components of the solar background, are often many orders of magnitude stronger than the EUV signal of interest. In the consumer sector, solar blind UV detectors would avoid the low efficiency and errors typical of infrared detectors when directly exposed to the solar light or strongly illuminated. NLOS communications in many consol applications that need to communicate with terminals that may intermittently become obscured by physical obstacles (play stations) could also be an area of application.
In recent years, a remarkable progress in the epitaxial growth technologies and in device processing of wide band gap semiconductors, including SiC, has been accomplished, which to date allows the fabrication of p-i-n, avalanche, metal-semiconductor-metal and Schottky barrier devices for UV detection. Photo-detectors use a low dark current, so high quality electronic materials may be necessary. Wide band gap semiconductors offer an ideal dark current that can be several orders of magnitude lower than silicon. In particular, 4H—SiC has an electronic quality that makes it reliably approach the ideal dark current even in relatively large diameter wafers. Moreover, as the only material among the known wide band gap semiconductors, 4H—SiC to have been tested for UV detection with the highest cutoff wavelength at 380 nm (Eg=3.26 eV) below the low end of the visible wavelength range (400 nm) and has been proved to be a very promising candidate for the development of visible-blind UV detectors [1].
Some publications have reported p-i-n and avalanche UV detectors realized on SiC with good UV photo detection [2]. For example, U.S. Pat. No. 5,093,576 to Edmond et al. discloses a pn junction photodiode formed in a silicon carbide substrate [3], and U.S. Pat. No. 7,002,156 to Sandvik et al. discloses a detection system including a SiC avalanche photodiode for use in harsh environments [4].
However, in order to enhance sensitivity at short wavelengths, metal-semiconductor (Schottky) diodes are preferred to p-i-n and junction diodes, as the carrier generation occurs in the space-charge region, i.e. at the semiconductor surface, with high built-in electric field. Moreover, Schottky diodes are majority carrier devices, thus allowing a faster response than p-n junctions. Finally, Schottky diodes typically have a simpler fabrication processes than p-i-n and junction structures. Metal-semiconductor photodiodes are particularly used for detection of high photon rates fluxes in the ultraviolet and visible-light regions. In fact, the absorption coefficients α in these regions is very high, of the order of 104 cm−1 or more, for most of the common semiconductors, which corresponds to an effective absorption length 1/α of 1.0 μm or less. However, in Schottky photodiodes to avoid large reflection and absorption losses when the diode is illuminated from the top, the metal film should to be very thin while antireflection coatings may be used to reduce the metal shining. A Metal-Semiconductor photodiode is disclosed in U.S. Pat. No. 4,763,176 to Masanori.
Typical Schottky-type SiC UV photodiodes use “semitransparent” continuous thin metal layers (<20 nm) with high values [1.4-1.8 eV] of the Schottky barrier on SiC (Ni, Au, Pt)[6]. However, these Schottky-type devices show a relatively low sensitivity in the wavelength range 200-250 nm, due to the low penetration depth of the UV radiation in the metal. In order to improve the quantum efficiency of SiC photo-detectors, one approach includes further reducing the thickness of the semitransparent film [7]. However, this approach can lead to a difficult control of the uniformity of an ultra thin Schottky barrier, beyond being detrimental for the mechanical and thermal stabilities of the contact.
The direct exposure to radiation of the optically active area, i.e. the depletion region of the junction, could be an alternative to improve the detector sensitivity at short wavelengths. With this aim, approaches such as planar p-i-n structures, with the intrinsic region directly exposed to radiation, or innovative semitransparent Schottky metallizations are actually under study. For example, metal-semiconductor-metal 4H—SiC photodiodes, using Ni/ITO interdigit contacts, were recently proposed by Chiou allowing the Schottky operation and the optically active area direct radiation exposure [6]. Finally, Sciuto et al. in [6] demonstrated high responsiveness 4H—SiC vertical Schottky UV photodiodes based on the pinch-off surface effect, obtained by way of self-aligned Nickel Silicide interdigit contacts. These planar detectors with the active area directly exposed to the optical flux allow an improvement of the sensitivity at short wavelengths [6].
The absorption coefficient of the electromagnetic radiation in silicon carbide decreases at a great rate for long wavelengths. Cha et al. in [8] disclose the absorption coefficient spectrum in 4H—SiC. As it can be seen in this reference, the absorption coefficient decreases at a great rate for wavelengths longer than 280-290 nm, due to the loss of efficiency of the absorption mechanisms (indirect transitions) for photons of these wavelengths. At 350 nm, for example, the absorption length of 4H—SiC is 35 μm [9], that is about the 67% of the incident optical flux at this wavelength is absorbed in an active layer thick 35 μm. This means that it may be necessary to increase as much as possible the thickness of the active area in silicon carbide photodiodes in order to enhance the absorption and consequently the detection efficiency at the high wavelength end of the sensitivity range of the material. On the other hand, the lower the wavelength, the higher the absorption coefficient.
The feasibility of deep trenches etched in lightly doped thick SiC epitaxial layers and filled with metal has been demonstrated in various publications. Abbondanza et al. [10] disclose a method for growing low defect and high thickness (up to 150 microns) silicon carbide epilayers onto silicon carbide substrates. Likewise, the etching of high aspect ratio deep trenches (>100 μm) in silicon carbide using Deep Reactive Ion Etching (DRIE), as well as time-multiplexed etch-passivation (TMEP) process, which alternates etching with polymer passivation of the etch sidewalls, has been reported by various authors (for example, Evans et al [11]). Finally, S. Grunow et al. [12] discloses a method for forming conductive structures by filling trenches with a metal.